Inductive versus steric effects

The reader may already have noted that the steric effects of the replacement of H atoms in CH3CI by an increasing number of Cl atoms, are excactly the opposite of the inductive effects expected from the larger electronegativity of Cl. 

The observed ClCCl valence angles in CH2CI2 and CHCI3, 112.3(1)° and 111.3(2)° respectively, indicate that valence angles are determined by steric Cl- Cl repulsions rather than the valence shell electron pair repulsions. 

In other respects inductive and steric effects appear to cancel out  

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The results mentioned are best demonstrated on secondary nitramines [7], which in their molecular structure are relatively simple polynitro compounds, and the mechanism of primary homolysis of their molecules is well understood [11,14] (Scheme 1). Polynitro arenes, on the other hand, have a more complex structure and intramolecular effects in their molecules here the mesomeric, inductive and steric effects on reactivity operate simultaneously. This fact makes the problem of their primary fragmentation somewhat complicated too [6,9] (Schemes 2, 3a, 3b, 7 and 8). If a molecule of these compounds contains several types of substituents, it can contain several potential reaction centres (e.g., the PYX and TMPM molecules, see Schemes 7 and 8). The initiation proper can then be realized by the molecule simultaneously participating by several centres or always by a single centre in a given type of initiation (the initiation of PYX by impact or shock versus its initiation by electric spark, see Figs. 12 and 26) [6]. 

Kinetic measurements on the dimeric hexamethoxydisiloxane species generated values of a = 0.0007 L/mol-min and kw = 0.0011 L/mol-min, versus 0.001 and 0.006 L/mol-min for the monomer (46). The alcohol-producing condensation reaction is about the same for monomers and dimers, but the water-producing condensation is significantly lower for dimers. This difference is probably due to both increased steric crowding in the dimer and the inductive effect of an -OSi ligand retarding the reaction, as described earlier. Steric and inductive effects are important variables on reaction kinetics. 

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